1. Field of the Invention
The present invention relates to fine wire feeding mechanisms of the type used on automatic semiconductor wire bonders. More particularly, the present invention relates to a wire tensioning apparatus used in a wire feed mechanism of an automatic wire bonder.
2. Description of the Prior Art
Heretofore, tensioning devices used to apply tension to a fine wire were known. Such fine wire feed mechanisms for fine wires used in wire bonders are classified in international Class B23K, 37/00 and U.S. Class 228/4.5.
Most of the basic problems associated with feeding fine wires have been recognized heretofore and some solutions have been provided to assure that the fine wire used in automatic wire bonders is isolated from its supply spool and is provided with a back tension or pull at the bonding tool. The fine wire is preferably isolated from a feed spool employing a wire feed slack loop device of the type shown and described in U.S. Pat. No. 4,736,826 which is assigned to the assignee of the present invention. The loop system shown and described in this patent is capable of applying a predetermined tension on a loop of wire leading from a wire spool, thus isolating the wire at the bonding tool from variations and spool movement or friction. Heretofore, tensioning devices have been employed to exert an additional tension force on the wire in a capillary bonding tool which is sufficient to seat a ball in the recess of the bonding tool at first bond. The tension force desired to seat the ball sometimes is in excess of the tension force desired at second bond and a compromise is made in this tension force.
Tests performed to determine a single optimum tension force at first bond have revealed that the operation of an automatic gold wire ball bonder can be enhanced by varying the tension of the wire above the capillary to improve the quality of the wire bonds at both first and second bond as well as increasing the mean time between bonding errors.
Accordingly, it would be desirable to provide a tensioning device that will virtually eliminate known problems associated with improper bonds made by automatic wire bonders.